(a) Field of the Invention
The present invention relates to an optical system for hard (or non-flexible) endoscopes and, more specifically, to an optical system in which one of plural relay lenses is designed as a variable-magnification lens system.
(b) Description of the Prior Art
In medical examinations through an endoscope, it is desired to magnify locations to be observed through the endoscope. In order to satisfy such a desire, it has conventionally been practised to displace the distal end of the endoscope from its normal position to another which is closer to the object to be observed and correct out-of-focus thereby caused by displacing the eyepiece in the optical system.
However, such a method has a defect that it does not permit observing images at a sufficiently high magnification for the following reasons: (1) it is impossible to displace the objective lens to a position sufficiently close to the object to be observed since the front focal point of the objective lens of the optical system is located inside the objective lens itself and (2) when the objective lens is displaced closer to the object to be observed, it is impossible to correct out-of-focus since movable range of the eyepiece is restricted.
As a second method to enable observation of images both at the normal size and a magnified size, it is practised to prepare objective lenses designed for different magnification levels and replace them with each other for observation. However, this method also has the following defects: (1) it is required to prepare additional objective lenses, (2) the endoscope requires tedious procedures to take it out of the body cavity for replacing the objective lens with another and the location to be observed may be lost out of sight when the endoscope is inserted once again into the body cavity and (3) the patient must suffer from much pain while the endoscope is inserted and taken out repeatedly.
A third method to observe a location at a magnified size is to use an eyepiece designed as a variable-magnification lens system in an endoscope. In such an endoscope, however, brightness is different on image at the normal size and one at magnified size since an image formed with the rays which have passed through the objective lens and relay lenses is magnified by the eyepiece. Therefore, the third method has a defect that images are darkened especially at the high magnification level required for minute observation.
In addition to the above-mentioned methods, it can be contrived to design an objective lens of an endoscope as a zoom lens system. However, a zoom lens system of the mechanical compensation type requires a complicated displacing mechanism and it is impossible to arrange a displacing mechanism in the distal end of an endoscope which must have a diameter as small as possible. Further, an objective lens system for an endoscope is very small and comprises practically no space that permits displacing specific lenses for zooming. It is therefore practically impossible to adopt a zoom lens system of optical compensation type as an objective lens system of an endoscope.